1. Field
The invention is directed to an industrial oven for curing composite material structures.
2. Description of the Related Art
Over the past two decades, military and commercial aircraft manufacturers have increased the use of composite materials in large aircraft. Among other things, composite materials tend to be lighter than conventional materials (e.g., riveted aluminum and honeycomb structures).
Composite material parts are typically made from multiple layers of “prepreg” (e.g., carbon or fiberglass fabric pre-impregnated with resin) which are draped in multiple directions over a steel, Invar, or aluminum mold (also called a “tool”). The structure is then “bagged,” which involves placing high-temperature bagging material over the part and sealing it to the tool surface (e.g., with flexible sealing putty called “tacky tape”). The structure is then cured, which serves to heat the composite material part (and resin) to a sufficiently temperature to melt the resin and cross-link the resin molecules to achieve a cured part. Typically, the final cure temperature is about 250° F. or 350° F. Temperature uniformity of the composite material structure during the curing process is very important because non-uniformity of temperature on the part can result in permanent warping of the structure, thereby changing the structural and aerodynamic stability of the part.
Aerospace autoclaves have been used to cure large composite material structures for aircraft and aerospace parts. An autoclave is a large pressure vessel that incorporates high pressure, high temperature, and vacuum to consolidate, bond and cure composite material structures. However, such autoclaves are expensive to install and operate, and require the installation of a pressurized nitrogen system to deliver pressure to the autoclave, which makes the use of autoclaves not cost effective for use in the manufacturing of large composite structural parts.
Industrial ovens have also been used to cure composite material structures for aircraft and aerospace parts. Such an oven is a large box-shaped unit that has typically been used to cure small non-structural composite material parts and operates by directing heated airflow over the composite material part in a side-to-side, side-to-top, bottom-to-top or top-to-side manner across the width of the oven. Such ovens, unlike autoclaves, operate at atmospheric pressure and so do not apply additional pressure to the composite material structure.
Because most high-performance resin systems require high pressure to achieve high-pressure curing, such ovens have not been deemed suitable for curing most structural composite material parts. Additionally, though such ovens have been used to cure small parts, which can then be bolted together to form larger structures, conventional ovens have various drawbacks that make them unsuitable for curing large parts (e.g., whole wings or fuselages of an aircraft) due to their large size and tight temperature uniformity considerations. Moreover, conventional ovens are not able to achieve uniform heating of such large parts, causing large temperature gradients across the parts that cause structural warping, nor able to heat the large tool and composite materials of aircraft parts (e.g., wings and fuselages) at the typical curing specification rates of 2-5° F./min.
Accordingly, there is a need for an industrial oven that overcomes the drawbacks noted above and can cure large composite material structures, such as aircraft and aerospace parts.